A modifying communication device includes multiple antenna elements that transmit signals to communicate information. A feedback communication device extracts information from the transmitted signals. The multiple antenna elements enhance spectral efficiency, allowing more users to be served simultaneously over a given frequency band. The transmitted signals, however, propagate along different paths and may reach the receiving communication device with different phases that destructively interfere. It is generally desirable to apply a relative phase between the two transmit signals to compensate for the phase difference due to the different paths or fading so that constructive interference can be achieved at the receiver.
The phase perturbation method may be used to derive the proper applied phase for obtaining constructive interference at the receiver. The phase perturbation method comprises perturbing the nominal value of the phase difference of two transmit diversity antennas continuously in alternating directions. Feedback information from the receiver is used to adjust the nominal valued of the phase difference so as to achieve constructive interference of the signals.
A problem with this technique is that ideally, the phase perturbation should only change the phase difference of the transmitted signals without affecting the phase of the combined received signal. Any discontinuity in the phase of the received signal is undesirable as it may interfere with the receiver channel estimation or SIR estimation and deteriorate the data decoding performance of the receiver.
As a solution to the disadvantages and problems associated with previous phase perturbation methods, a technique has been suggested for modifying signals using symmetrical phase perturbation. In symmetric perturbation the changes in the phase difference are accomplished by adjusting the phase of one antenna by half the desired phase difference change in one direction and the phase of the other antenna by half the desired phase difference change in the opposite direction. Symmetric perturbation results in less phase discontinuity for the combined (i.e. beam-formed) signal than asymmetric perturbation (where all the desired phase difference change is applied to one of the antennas only).
The phase discontinuity can be further reduced by reducing the size of the perturbation, i.e. by creating smaller changes in the phase difference, and/or by doing the perturbation less frequently. In either case the perturbation may become less efficient, in particular under fast changing environment conditions.
The perturbation technique may be applied with specific system independent quality indicators such as the power control of CDMA and WCDMA systems. This approach may have the advantage of being transparent to the network and have little or substantially no impact on the specific interface as defined by the standards.